1. Field of the Invention
The present invention relates to a surface mount type surface acoustic wave (hereinafter referred to as “SAW”) filter incorporating a SAW chip, which is an electronic component making use of SAW on a piezoelectric substrate and, more particularly, to their measures for radiating heat.
2. Description of the Related Art
In recent years, SAW filters for selectively passing signals of particular frequencies therethrough are in extensive use as transmitting filters or receiving filters in antenna switching circuits or the like for transmitting and receiving high-frequency signals.
FIG. 3 is a sectional view illustrating a conventional construction of one of the aforesaid SAW filters. Referring to the figure, the conventional SAW filter is primarily constructed of a multi-layer substrate (LTCC) 1, a SAW chip 2 mounted in a recession (i.e., a recess) 1a of the multi-layer substrate 1, an end surface electrode 3 extending from a side surface of the multi-layer substrate 1 toward a bottom surface thereof, and a sealing plate 4 formed of a metal plate or the like connected at a position appropriate to cover the recession 1a of the multi-layer substrate 1. The multi-layer substrate 1 is composed of layered green sheets made of a low-temperature sintered ceramic material and fired at 10000° C. or less. The multi-layer substrate 1 is mounted on a motherboard (not shown). The SAW chip 2 is formed of a piezoelectric substrate made of crystal or the like, a predetermined electrode pattern being formed on the surface thereof. The SAW chip 2 is connected to a lead pattern 6 by a bonding wire 5. The lead pattern 6 is in conduction with the end surface electrode 3, so that the SAW chip 2 can be electrically connected to an external circuit through the intermediary of the end surface electrode 3. The bottom surface of the SAW chip 2 is adhesively secured to the inner bottom surface of the recession 1a of the multi-layer substrate 1, and the recession 1a is filled with an inert gas, such as nitrogen.
When a SAW filter is used as a transmitting filter for an antenna switching circuit, a transmitter signal amplified by a power amplifier is supplied to the SAW filter. Hence, the heat generated by the SAW chip 2 shown in FIG. 3 cannot be ignored. More specifically, when an amplified transmitter signal is received, the SAW chip 2 generates considerable Joule heat; therefore, if transmission is continued for a prolonged time, the mere heat radiation primarily depending upon the sealing plate 4 can no longer sufficiently restrain a temperature rise in the SAW chip 2. For this reason, the conventional SAW filters used as transmitting filters have been posing a problem in that an undue temperature rise attributable to the heat generated by themselves adversely affects the characteristics of the SAW chip 2, making it difficult to achieve high reliability thereof.